Trinanjan Datta

Spin transport in the Néel and collinear antiferromagnetic phase of the two dimensional spatial and spin anisotropic Heisenberg model on a square lattice

AbstractWe analyze and compare the effect of spatial and spin anisotropy on spin conductivity in a two dimensional Sxa0=xa01/2 Heisenberg quantum magnet on a square lattice. We explore the model in both the Néel antiferromagnetic (AF) phase and the collinear antiferromagnetic (CAF) phase. We find that in contrast to the effects of spin anisotropy in the Heisenberg model, spatial anisotropy in the AF phase does not suppress the zero temperature regular part of the spin conductivity in the zero frequency limit–rather it enhances it. In the CAF phase (within the non-interacting approximation), the zero frequency spin conductivity has a finite value, which is suppressed as the spatial anisotropy parameter is increased. Furthermore, the CAF phase displays a spike in the spin conductivity not seen in the AF phase. We also explore the finite temperature effects on the Drude weight in the AF phase (within the collisionless approximation). We find that enhancing spatial anisotropy increases the Drude weight value and increasing spin anisotropy decreases the Drude weight value. Based on these studies, we conclude that antiferromagnets with spatial anisotropy are better spin conductors than those with spin anisotropy at both zero and finite temperatures.n

Non-linear spin wave theory results for the frustrated S=\frac {1}{2} Heisenberg antiferromagnet on a body-centered cubic lattice

At zero temperature the sublattice magnetization of the quantum spin- 1/2 Heisenberg antiferromagnet on a body-centered cubic lattice with competing first and second neighbor exchange (J(1) and J(2)) is investigated using the non-linear spin wave theory. The zero temperature phases of the model consist of a two sublattice Néel phase for small J(2) (AF(1)) and a collinear phase at large J(2) (AF(2)). We show that quartic corrections due to spin wave interactions enhance the sublattice magnetization in both the AF(1) and the AF(2) phase. The magnetization corrections are prominent near the classical transition point of the model and in the J(2)>J(1) regime. The ground state energy with quartic interactions is also calculated. It is found that up to quartic corrections the first order phase transition (previously observed in this model) between the AF(1) and the AF(2) phase survives.

Zero Temperature Phases of the Frustrated J1–J2 Antiferromagnetic Spin-1/2 Heisenberg Model on a Simple Cubic Lattice

At zero temperature magnetic phases of the quantum spin-1/2 Heisenberg antiferromagnet on a simple cubic lattice with competing first and second neighbor exchanges (J1 and J2) is investigated using the non-linear spin wave theory. We find existence of two phases: a two sublattice Néel phase for small J2 (AF), and a collinear antiferromagnetic phase at large J2 (CAF). We obtain the sublattice magnetizations and ground state energies for the two phases and find that there exists a first order phase transition from the AF-phase to the CAF-phase at the critical transition point, pc=0.56 or J2/J1=0.28. We also show that the quartic 1/S corrections due spin-wave interactions enhance the sublattice magnetization in both the phases which causes the intermediate paramagnetic phase predicted from linear spin wave theory to disappear.

Effects of magnetic field, anisotropy, and biquadratic interactions in type-IIA fcc antiferromagnets studied by linear spin-wave theory

State Key Lab of Optoelectronic Materials and Technologies,School of Physics and Engineering, Sun Yat-sen University, Guangzhou 510275, China(Dated: September 13, 2011)We study the spin dynamics in a 3D quantum antiferromagnet on a face-centered cubic (FCC)lattice. The e ects of magnetic eld, single-ion anisotropy, and biquadratic interactions are investi-gated using linear spin wave theory with Dyson-Maleev transformation for spins in a canted basis.We calculate the expected nite frequency neutron scattering intensity and give qualitative criteriafor typical FCC materials MnO and CoO. The magnetization reduction due to quantum zero pointuctuations is also analyzed.PACS numbers: 75.30.Ds, 75.10.Jm, 75.50.EeI. INTRODUCTION

The ground state phase diagram of the quantum J1–J2 spin-1/2 Heisenberg antiferromagnet on an anisotropic square lattice

We have studied the ground state phase diagram of the quantum spin-1/2 frustrated Heisenberg antiferromagnet on a square lattice by using the framework of the differential operator technique. The Hamiltonian is solved by using an effective-field theory for a cluster with two spins (EFT-2). The model is described using the Heisenberg Hamiltonian with two competing antiferromagnetic interactions: nearest neighbor (NN) with different coupling strengths J1 and J1 along the x and y directions and next nearest neighbor (NNN) with coupling J2. We propose a functional for the free energy (similar to the Landau expansion) and using Maxwell construction we obtain the phase diagram in the (λ, α) space, where λ = J1/J1 and α = J2/J1. We obtain three different states depending on the values of λ and α: antiferromagnetic (AF), collinear antiferromagnetic (CAF) and quantum paramagnetic (QP). For an intermediate region λ1 < λ < 1 we observe a QP state between the ordered AF and CAF phases, which disappears for λ above some critical value . We find a second-order phase transition between the AF and QP phases and a first-order transition between the CAF and QP phases. The boundaries between these ordered phases merge at the quantum triple point (QTP). Below this QTP there is again a direct first-order transition between the AF and CAF phases, with a behavior approximately described by the classical line .

1D FFLO state in absence of time reversal symmetry breaking

We propose a route to a one-dimensional Fulde-Ferrell-Larkin-Ovchinnikov state in the absence of broken time-reversal symmetry. At present such a state may be encouraged in a clean (no disorder) AlAs quantum wire fabricated using the cleaved edge overgrowth technique. The fabrication technique captures two degenerate nonoverlapping bands separated in momentum-space by half an umklapp vector which leads to four Fermi points. Using field theoretic methods such as abelian bosonization and the renormalization group scheme we treat the important low energy long wavelength fermionic interaction terms for this one dimensional system. Due to the specific bandstructure arrangement of the quantum wire there is a new class of unique umklapp assisted interactions. These umklapp interactions are present at all electronic densities and are not related to the commensurability of the electron gas with the underlying lattice. We show that in the presence of the umklapp interactions and without any external perturbations such as a magnetic or electric field a singlet superconducting ground state is preferred with non-zero center-of-mass momentum for the Cooper pairs. The finite pairing momentum of the Cooper pairs is an indication of a Fulde-Ferrell-Larkin-Ovchinnikov state which is known to lead to inhomogeneous superconductivity.

Exact solution and high-temperature series expansion study of the one-fifth-depleted square-lattice Ising model.

The critical behavior of the one-fifth-depleted square-lattice Ising model with nearest-neighbor ferromagnetic interaction has been investigated by means of both an exact solution and a high-temperature series expansion study of the zero-field susceptibility. For the exact solution we employ a decoration transformation followed by a mapping to a staggered eight-vertex model. This yields a quartic equation for the critical coupling giving K(c)(≡βJ(c))=0.695. The series expansion for the susceptibility, to O(K(18)), when analyzed via standard Padé approximant methods gives an estimate of K(c), consistent with the exact solution result to at least four significant figures. The series expansion is also analyzed for the leading amplitude and subdominant terms.

Indirect K-edge bimagnon resonant inelastic x-ray scattering spectrum of α-FeTe

We calculate the K-edge indirect bimagnon resonant inelastic x-ray scattering (RIXS) intensity spectra of the bicollinear antiferromagnetic order known to occur in the α-FeTe chalcogenide system. Utilizing linear spin wave theory for this large-S spin system we find that the bimagnon spectrum contains four scattering channels (two intraband and two interband). We find from our calculations that for suitable energy-momentum combination the RIXS spectra can exhibit a one-, two- or three- peak structure. The number of peaks provides a clue on the various bimagnon excitation processes that can be supported both in and within the acoustic and optical magnon branches of the bicollinear antiferromagnet. Unlike the RIXS response of the antiferromagnetic or the collinear antiferromagnetic spin ordering, the RIXS intensity spectrum of the bicollinear antiferromagnet does not vanish at the magnetic ordering wave vector [Formula: see text]. It is also sensitive to next-next nearest neighbor and biquadratic coupling interactions. Our predicted RIXS spectrum can be utilized to understand the role of multi-channel bimagnon spin excitations present in the α-FeTe chalcogenide.